This invention relates to an electromagnetically compensating weighing or force-measuring device with a working coil movable in an air gap of a fixed magnetic circuit and influenceable by the load to be measured. The invention relates, more particularly, to such a device having an indicator for indicating the position of the working coil and also having a control amplifier coupled to the indicator. The working coil is fed by an output signal from the control amplifier and is thereby brought to the zero position.
In electromagnetic force compensation of the type mentioned above, the working coil, subjected to the influence of the load to be measured, is held in a zero position by controlling the current flowing through the working coil, so that the current I flowing through the coil, by the formula EQU F=B.times.I.times.n.times.2.pi.r
constitutes a measure of the electromagnetically produced force F and consequently the load to be measured. In the equation set out above: B is the magnetic flux density of the magnet system; I is the current in the coil; n is the number of turns on the working coil; and 2.pi.r is the length of one turn of the working coil.
In the above set out formula, it is generally assumed that the magnetic flux density B is constant, so that the current I in the working coil is directly proportional to the electromagnetically generated force.
In general, the coil current I is converted into a measured voltage in a measuring resistance, and this measured voltage is then converted into digital form by means of an analog/digital converter, by comparing it with an internal reference voltage.
In such an arrangement, a permanent magnet is generally used as the principal component the magnetic circuit, while the reference voltage is generally provided by a reference diode, such as a Zener diode.
In such an arrangement, a number of difficulties and shortcomings arise. These are:
1. The temperature coefficient and the aging of the magnet falsify the measurement results; PA1 2. The temperature coefficient and the aging of the measurement resistance also falsify the measurement result; the resultant systematic error having a particularly pronounced negative influence on measurement, because appropriately varying currents must be used for compensation of varying loads, whereby a varying amount of heat production and hence various temperature increases are produced; and PA1 3. The temperature coefficient of the reference diode and hence the reference voltage as well as the aging of the reference diode, also falsify the measurement result, in addition, statistical voltage discontinuities in the reference diode cannot be avoided, and they likewise can influence measurement in an uncontrollable manner.
In addition to simple temperature compensation by additional temperature-dependent components, various possibilities are known for eliminating some or all of these possible errors in a systematic fashion.
One possibility is "quotient measurement". Two conventional systems are used simultaneously, the first system being subject to the influence of the load being measured and the second system being subject to the influence of a constant mass. In this manner, the second system replaces the reference diode.
In an embodiment according to German Federal Republic Pat. No. 1,194,167, the two conventional systems are disposed in two respective gaps of a magnetic circuit which includes the permanent magnet.
In another embodiment, disclosed in U.S. Pat. No. 3,322,222, the two conventional systems are located in the same gap.
In a device disclosed in Swiss Pat. No. 521,575, the magnetic flux density B is regulated by the second conventional system.
Finally, according to another proposal, the two conventional systems are guided by concentric cylinders with the aid of air cushions.
By using the solution provided by the above-mentioned dual systems, the two above-mentioned difficulties (1) and (3) are overcome, because these influences act upon the two systems in similar fashion, and hence can be eliminated by quotient formation. The above-mentioned problems and difficulties (2), however, nevertheless remain because the measurement resistances for the two systems are separate. Moveover, a device using the dual systems, quotient formation approach has a very complex design, since all of the individual parts must be provided in duplicate. Furthermore, such a device is expensive. Finally, changes in the zero setting of the second conventional system may not be recognized or even detected, so that the dual systems in use may falsify the measurement result.
In a modification of the aforementioned quotient measurement technique, a digitalization is also carried out, wherein the carrier current, in other words the current I flowing through the working coil, is broken up into pulses in the first system. In an embodiment according to German Federal Republic Pat. No. 1,194,167, the pulse frequency is proportional to the applied mass, while in a modification disclosed in Swiss Pat. No. 529,999 the pulse length is proportional to the applied mass.
In this fashion, the above-mentioned difficulty, designated by the numeral (2), can be elimitated, but the entire device still has a very complex structure and requires a large number of components because the second system must still be present.
Moreover, changes in the zero point of the second system still cannot be detected or recognized, thus leading to possible falsifying of the measurement result. Finally, a complex electronic circuit subject to the influence of temperature and aging is required to keep the pulse constant. In particular, the shape of the edges changes markedly with minor changes in the components. If the pulse shape is not kept constant, accurate measurement is no longer possible.
Finally, German Federal Republic Auslageschrift (Published Patent application) No. 2,511,103 discloses a digitalization in the control circuit. However, this eliminates only the problems described above under numerals (1) and (2), while the difficulty described above under the numeral (3) remains. Once again, this can be overcome using the prior art techniques only by resorting to the dual systems and effecting a quotient measurement. Thus, disadvantages remain. Moreover, the characteristic curve and the hysteresis of the soft-magnetic material in the magnetic circuits used constitute critical parameters.